Latent heat of vapourisation

• The quantity of heat required to convert one unit mass of a liquid into vapour at its boiling point without any change of temperature is called its latent heat of vapourisation.

Latent heat of vapourisation =

Latent heat of vapourisation of water:

• The latent heat of vapourisation of water is the amount of net required to convert one unit mass of water into vapour at its boiling point (i.e., at 100ºC at 1 atm pressure)

Latent heat of vapourisation of water = 22.6 × 10⁵ J/kg

• The heat energy supplied to a boiling liquid is used up in changing the state of the liquid substance by overcoming the force of attraction between its particles.
• Water is a liquid substance. There is cohesive force between the particles of water. These forces of attraction hold the water particles together in the liquid state.
• The heat which is supplied to water during boiling is used to overcome (or break) the force of attraction between water particles so that they become totally free and change into a gas (Steam).
• The heat supplied does not increase the temperature of boiling water.

Natural consequences of high latent heat of vapourisation of water:

• The Latent heat of vapourisation of water is high (22.6 × 10⁵ J/kg) thus 2,260,000 J of heat is released when 1 kg of steam at 100ºC condenses into water at same temperature.
• The energy released during condensation of steam is used in many ways:

1. For running trains and industrial machines.
2. For generation electricity in thermal and nuclear power plants.
3. For steam pressing in laundries.

Steam at 100ºC cause more severe burns than hot water at 100ºC:

Reason:

• Steam at 100ºC has more heat in the form of latent heat than the boiling water so when steam comes in contact with the skin, it releases more heat than the boiling water at 100ºC.
• When steam condenses it releases heat to the surroundings. When just 1 mL of steam water condenses it releases 2.3 × 10³ J of heat. Then the condensed water cools from 100⁰C to 37⁰C (body temperature), so even more heat is released and absorbed by the skin. That's why a steam burn is much worse than a burn from boiling water.

Cooling Off When Wet

• We can cool off our body by just coming out of a shower or swimming pool if we do not dry off with a towel. It takes 2.3 × 10⁶ J of heat to vaporize 1 kg of water. When we are wet, heat from our body is used to vapourise the water. Because heat leaves our body, we feel cool. If we don't want to cool off, we can just dry ourselves well with a towel.